It has been revealed in recent years that fibers of high strength and high modulus of elasticity can be produced by melt-spinning an aromatic polyester that exhibits anisotropy in its molten state. This process has various advantages in that no solvent is used and conventional spinning apparatuses can be used in the process. Although such fiber forms a highly oriented and highly crystalline structure and exhibits excellent properties merely by melt-spinning, both the strength and the modulus of elasticity of the fiber can be further improved when the fiber is heat-treated in the vicinity of its softening temperature. However, some fusion among filaments is likely to occur in heat treatment; and once when fusion occurs, since a stress along the fiber axis direction is partly converted to a stress in a direction perpendicular to the fiber axis, the resulting fiber will have a defect characteristic of aromatic polyester fiber in a magnified extent in that although it shows high strength in the fiber axis direction it is eventually very brittle in the direction perpendicular to the fiber axis owing additionally to a weak cohesive force between molecules.
Accordingly, there has been eagerly awaited a means for overcoming the problem of fusion occurring in heat treatment. However, no practically useful means has ever been found yet.